The early mechanical advance distributors and ignitions were never designed to have much a curve.

The weights simply flop to full advance just above idle speed. The stock weights are much too heavy and the springs are too light. This also causes the weight stop pins and pin holes to wear rapidly in the advanced direction. This has been the death of many tops ends due to excessive heat.

Mechanical advance ignitions are custom re-curved by cutting down the fly weights, re-pinning and installing extra heavy springs to achieve an advance curve that best matches the engine combination.

The advance curve is then tested on a Sun distributor machine adapted to test Harley-Davidson® ignitions.